An engine may include a compression ratio changing mechanism to increase engine efficiency at low engine loads and reduce the possibility of engine knock at higher engine loads. The compression ratio changing mechanism may change a height of a piston, change connecting rod length, or change orientation of a connecting rod to change a cylinder clearance volume at top dead center compression stroke. The cylinder's compression ratio may be dynamically changed as engine speed and engine load change. However, it may be possible for the compression ratio changing mechanism to degrade such that it may not change a compression ratio of a cylinder in a desirable way. Further, it may be desirable to determine whether or not the compression ratio changing mechanism is operating as expected, but vehicle occupants may find it objectionable to change the engine's compression ratio when it may lead to engine knock or a noticeable driveline torque change. Therefore, it may be desirable to diagnose operation of a compression ratio changing mechanism in a way that may not interfere with vehicle operation.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for operating an engine, comprising: adjusting poppet valve timing and sampling of a pressure via a controller in response to a request to diagnose a variable compression ratio mechanism.
By adjusting poppet valve timing and pressure sampling, it may be possible to diagnose operation of a compression ratio changing mechanism in a way that may not disturb vehicle occupants. In one example, poppet valve timing may be advanced from base timing to improve a signal to noise ratio of an intake manifold pressure or an exhaust manifold pressure so that an engine's compression ratio changing mechanism may be diagnosed as degraded or functioning as intended. Further, the diagnostic may be performed during deceleration fuel shut off (DFSO) when the engine is not producing torque so that driveline torque disturbances may be reduced and so that a signal to noise ratio may be improved.
The present description may provide several advantages. Specifically, the approach may diagnose operation of a cylinder compression changing mechanism without making vehicle occupants aware that the compression changing mechanism is being diagnosed. In addition, the approach may be performed with existing sensors so that system cost may not increase. Further, the approach may diagnose more than two operating states of the compression changing mechanism.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.